.(See description below)
.
Milky Way Galaxy – an Illustration
For decades, astronomers have been blind to what our galaxy, the
Milky Way, really looks like. After all, we sit in the midst of it and can't
step outside for a bird's-eye view. Now, new images from NASA's Spitzer Space Telescope are shedding light on the
true structure of the Milky Way, revealing that it has just two major arms of
stars instead of the four it was previously thought to possess.
Since the 1950s, astronomers have produced maps of the Milky Way. The early
models were based on radio observations of gas in the galaxy, and suggested a
spiral structure with four major star-forming arms, called Norma,
Scutum-Centaurus, Sagittarius and Perseus. In addition to arms, there are bands
of gas and dust in the central part of the galaxy. Our sun lies near a small,
partial arm called the Orion Arm, or Orion Spur, located between the Sagittarius
and Perseus arms.
Large infrared sky surveys in the 1990s led to some major revisions of these
models, including the discovery of a large bar of stars in the middle of the
Milky Way. Infrared light can penetrate through dust, so telescopes designed to
pick up infrared light get better views of our dusty and crowded galactic
center. In 2005, Benjamin and his colleagues used Spitzer's infrared detectors
to obtain detailed information about our galaxy's bar, and found that it extends
farther out from the center of the galaxy than previously thought.
The team of scientists now has new infrared imagery from Spitzer of an expansive
swath of the Milky Way, stretching 130 degrees across the sky and one degree
above and below the galaxy's mid-plane.
The findings make the case that the Milky Way has two major spiral arms, a
common structure for galaxies with bars. These major arms, the Scutum-Centaurus
and Perseus arms, have the greatest densities of both young, bright stars, and
older, so-called red-giant stars. The two minor arms, Sagittarius and Norma, are
filled with gas and pockets of young stars. Benjamin said the two major arms
seem to connect up nicely with the near and far ends of the galaxy's central
bar.
Though galaxy arms appear to be intact features, stars are actually constantly
moving in and out of them as they orbit the center of the Milky Way, like London
commuters in a busy traffic circle. Our own sun might have once resided in a
different arm. Since it was formed more than 4 billion years ago, it has
traveled around the galaxy 16 times.